4,224 research outputs found
An approximation function for frequency constrained structural optimization
The purpose is to examine a function for approximating natural frequency constraints during structural optimization. The nonlinearity of frequencies has posed a barrier to constructing approximations for frequency constraints of high enough quality to facilitate efficient solutions. A new function to represent frequency constraints, called the Rayleigh Quotient Approximation (RQA), is presented. Its ability to represent the actual frequency constraint results in stable convergence with effectively no move limits. The objective of the optimization problem is to minimize structural weight subject to some minimum (or maximum) allowable frequency and perhaps subject to other constraints such as stress, displacement, and gage size, as well. A reason for constraining natural frequencies during design might be to avoid potential resonant frequencies due to machinery or actuators on the structure. Another reason might be to satisy requirements of an aircraft or spacecraft's control law. Whatever the structure supports may be sensitive to a frequency band that must be avoided. Any of these situations or others may require the designer to insure the satisfaction of frequency constraints. A further motivation for considering accurate approximations of natural frequencies is that they are fundamental to dynamic response constraints
Implementation of generalized optimality criteria in a multidisciplinary environment
A generalized optimality criterion method consisting of a dual problem solver combined with a compound scaling algorithm was implemented in the multidisciplinary design tool, ASTROS. This method enables, for the first time in a production design tool, the determination of a minimum weight design using thousands of independent structural design variables while simultaneously considering constraints on response quantities in several disciplines. Even for moderately large examples, the computational efficiency is improved significantly relative to the conventional approach
Intrinsic pinning on structural domains in underdoped single crystals of Ba(FeCo)As
Critical current density was studied in single crystals of
Ba(FeCo)As for the values of spanning the entire doping
phase diagram. A noticeable enhancement was found for slightly underdoped
crystals with the peak at . Using a combination of polarized-light
imaging, x-ray diffraction and magnetic measurements we associate this behavior
with the intrinsic pinning on structural domains in the orthorhombic phase.
Domain walls extend throughout the sample thickness in the direction of
vortices and act as extended pinning centers. With the increasing domain
structure becomes more intertwined and fine due to a decrease of the
orthorhombic distortion. This results in the energy landscape with maze-like
spatial modulations favorable for pinning. This finding shows that iron-based
pnictide superconductors, characterized by high values of the transition
temperature, high upper critical fields, and low anisotropy may intrinsically
have relatively high critical current densities.Comment: estimation of Jc correcte
Single crystal growth and characterization of the large-unit-cell compound Cu13Ba
Single crystals of CuBa were successfully grown out of Ba-Cu self
flux. Temperature dependent magnetization, , electrical resistivity,
, and specific heat, , data are reported. Isothermal
magnetization measurements, , show clear de Haas-van Alphen oscillations
at = 2 K for applied fields as low as = 1T. An anomalous behavior
of the magnetic susceptibility is observed up to ~ 50K reflecting the
effect of de Haas-van Alphen oscillations at fairly high temperatures. The
field- and temperature-dependencies of the magnetization indicate the presence
of diluted magnetic impurities with a concentration of the order of 0.01at.%.
Accordingly, the minimum and lower temperature rise observed in the electrical
resistivity at and below = 15K is attributed to the Kondo impurity effect.Comment: 6 pages, 8 figures, accepted for publication in J. Alloys Comp
Direct observation of Fe spin reorientation in single crystalline YbFe6Ge6
We have grown single crystals of YbFe6Ge6 and LuFe6Ge6 and characterized
their anisotropic behaviour through low field magnetic susceptibility,
field-dependent magnetization, resistivity and heat capacity measurements. The
Yb+3 valency is confirmed by LIII XANES measurements. YbFe6Ge6 crystals exhibit
a field-dependent, sudden reorientation of the Fe spins at about 63 K, a unique
effect in the RFe6Ge6 family (R = rare earths) where the Fe ions order
anti-ferromagnetically with Neel temperatures above 450 K and the R ions'
magnetism appears to behave independently. The possible origins of this unusual
behaviour of the ordered Fe moments in this compound are discussed.Comment: 12 pages, 8 figures, accepted in J. Phys.: Cond. Matte
Non-conventional superconducting fluctuations in Ba(Fe1-xRhx)2As2 iron-based superconductors
We measured the static uniform spin susceptibility of
Ba(FeRh)As iron-based superconductors, over a broad range
of doping () and magnetic fields. At small fields ( 1 kOe) we observed, above the transition temperature , the occurrence
of precursor diamagnetism, which is not ascribable to the Ginzburg-Landau
theory. On the contrary, our data fit a phase fluctuation model, which has been
used to interpret a similar phenomenology occurring in the high- cuprate
superconductors. On the other hand, in presence of strong fields the
unconventional fluctuating diamagnetism is suppressed, whereas 3D fluctuations
are found, in agreement with literature
Can the US Minimum Data Set Be Used for Predicting Admissions to Acute Care Facilities?
This paper is intended to give an overview of Knowledge Discovery in Large Datasets (KDD) and data mining applications in healthcare particularly as related to the Minimum Data Set, a resident assessment tool which is used in US long-term care facilities. The US Health Care Finance Administration, which mandates the use of this tool, has accumulated massive warehouses of MDS data. The pressure in healthcare to increase efficiency and effectiveness while improving patient outcomes requires that we find new ways to harness these vast resources. The intent of this preliminary study design paper is to discuss the development of an approach which utilizes the MDS, in conjunction with KDD and classification algorithms, in an attempt to predict admission from a long-term care facility to an acute care facility. The use of acute care services by long term care residents is a negative outcome, potentially avoidable, and expensive. The value of the MDS warehouse can be realized by the use of the stored data in ways that can improve patient outcomes and avoid the use of expensive acute care services. This study, when completed, will test whether the MDS warehouse can be used to describe patient outcomes and possibly be of predictive value
Hydrostatic and uniaxial pressure dependence of superconducting transition temperature of KFe2As2 single crystals
We present heat capacity, c-axis thermal expansion and pressure dependent,
low field, temperature dependent magnetization for pressures up to ~ 12 kbar,
data for KFe2As2 single crystals. Tc decreases under pressure with dTc/dP ~
-0.10 K/kbar. The inferred uniaxial, c-axis, pressure derivative is positive,
dTc/dpc ~ 0.11 K/kbar. The data are analyzed in comparison with those for
overdoped Fe-based superconductors. Arguments are presented that
superconductivity in KFe2As2 may be different from the other overdoped,
Fe-based materials in the 122 family
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